1. Field of the invention
The present invention relates to a blood pump.
2. Description of the Prior Art
Researches on artificial hearts have been made worldwide. The artificial hearts subject to these researches are classified largely into two: one is a complete artificial heart which perfectly replaces the heart of a patient and the other is a supplementary artificial heart or blood pump which is used temporarily to assist the heart and to sustain the life of a patient, and which is removed, after restoring the function of his heart. Typically, they are used for ventricular assist. (Hereinafter, these two types of artificial hearts are generally called "artificial heart".) The researches on the artificial hearts have been actively underway in terms of function and safety. In particular, the antithrombosis materials are one of the essential factors in development of the artificial hearts. It is not going too far to say that development of the artificial hearts has been dependent upon development of antithrombosis materials. In addition to antithrombosis property, the mechanical properties of the materials are very important. Another important factor is the durability of heart valves which play a key role in the artificial heart.
The artificial heart must be characterized by the same function as that of a natural heart. In other words, the artificial heart must be designed so as to discharge blood at a pulsation similar to that of a natural heart. Therefore, the artificial heart is equipped with ports incorporating a valve. The artificial heart consists of at least a capacity-variable blood chamber, a blood introduction port and a blood discharge port which are connected with the blood chamber. Each of the blood introduction port and the blood discharge port has a valve preventing a counter flow of blood. When the blood chamber expands, the blood introduction valve opens and the blood discharge valve closes. On the other hand, when the blood chamber contracts, the blood introduction valve closes and the blood discharge valve opens so as to discharge blood through the blood discharge valve.
According to the differences in shape of a blood chamber and in contraction and expansion process, the artificial hearts can be classified into sack type, tube type, diaphragm type, pusher plate type, etc. The mechanism to introduce and discharge blood is as mentioned above and there remains no particular difference among the types.
So far there have been a variety of studies to determined what power is applied to the valves. As for a valve, for example, a disk-type Bjork-Shiley valve, a blood chamber sufficiently filled with blood contracts instantaneously by an external pressure (normally 100 to 300 mm Hg), thereby pushing the discharge valve open so as to discharge instantaneously blood against aorta pressure. Then the stent (a valve support rod) crashes into the valve disk. When the blood chamber expands after discharging blood, the pressure in the blood chamber becomes negative and a pressure of 100 to 300 mm Hg then formed on the artery side is applied smashingly to the valve in the adverse direction. This forms a blood hammer, which strikes the valve. (In general, such an impact liquid force is called "water hammer".) The severe shock like this is repeated as much as the pulsation of a heart, that is, approx. 100,000 beats daily, approx. 3 million beats monthly or approx. 40 million beats yearly. This phenomenon can be seen also in the blood introduction valve. When the blood chamber of an artificial heart expands, this valve opens with a pressure of approx. 50 mm Hg at the left atrium. Nevertheless, when the blood chamber contracts, a pressure of 100 to 300 mm Hg is applied smashingly in the adverse direction. This shock to the valve is applied to the valve disk and also to the valve-supporting stent. The frequency of shock amounts to 100,000 or more, hence making it difficult to take adequate safety measures against breakage of valves due to shock. Artificial heart valves now commercialized for replacement of natural ones can work as long as about 5 years, or can open and close at least 200 million times. Most of the artificial hearts so far developed around the world employ such commercialized artificial heart valves for the purpose of replacing natural ones from a safety viewpoint.
Very interesting and very important is it in that an artificial heart valve which has been quite safe in replacement of a natural one, may cause many unexpected troubles, for example, removal of a valve disk, damage of a disk and stent, when it is incorporated in an artificial heart.